A ball screw having a screw shaft, a nut and a plurality of balls rollably therebetween is capable of reducing friction resistance caused in rotating the screw shaft relative to the nut as compared with a sliding contact type screw. Therefore, the ball screw is commercially practical in various fields, such as machine positioning mechanisms, feed mechanisms, vehicle steering gears and the like. In such a ball screw, when the screw shaft is rotated relative to the nut, each ball rolls on a loaded ball rolling path between the nut and the screw shaft. Once rolling up to one end of the loaded ball rolling path, the ball goes into a ball return path formed in the nut, rolls on the ball return path and then returns to the loaded ball rolling path again.
When balls circulates inside the nut as described above, the balls may come into contact with each other or the inner peripheral surface of the ball return path, which causes noise. In order to reduce noise of the ball screw, there are known a technique of interposing a retainer between balls thereby to prevent the balls from coming into contact with each other and a technique of providing a noise insulating member over a return pipe of the ball return path.    [Patent Document 1] Japanese Patent Laid-open Publication No. 2005-308081